Printing presses are high-throughput, high-precision machines. Modern sheet-fed presses may operate at speeds in excess of 18,000 sheets per hour (five sheets per second). These multi-color presses rely on a variety of rollers and cylinders machined to very close tolerances in order to produce high resolution imagery. Each color of ink used on a sheet-fed press has its own dedicated set of rollers and cylinders to distribute and apply the ink to the substrate. During normal operation, all types of printing inks including “conventional,” “hybrid,” and “UV-sensitive” types can quickly build up on these cylinders, particularly just outboard the substrate perimeter. Each time a sheet of substrate passes by, another miniscule layer of ink is usually applied to the cylinder as well. Eventually a noticeable build-up of ink forms on the cylinder. A build-up of ink only several thousandths of an inch thick can often be enough to cause an unacceptable reduction of image resolution and can also result in unwanted contamination of subsequent substrate being processed.
Volatile Organic Compounds (VOCs) are nominally identified as the family of carbon-based compounds that exhibit certain evaporation rates and/or certain photochemical reaction rates, especially those that are known to contribute to air, water, and soil pollution, and/or those that may incur serious health risks.
Conventional ink has various amounts of volatile solvents in its chemical make-up. As these solvents evaporate from the ink, the ink dries. Hybrid inks are made up of a mixture of conventional ink and UV-curable compounds, and cure partially by solvent evaporation and partially by UV-induced cross-polymerization. Both of these types of inks usually require the use of volatile solvents for clean-up. Standard equipment on printing presses includes fume hoods and other venting equipment to remove solvent vapors from the vicinity of the press and out of the building housing the press.
UV-sensitive inks typically have less than 1% VOCs. For practical purposes these inks are considered to be “VOC-free” and do not depend on solvent evaporation to dry or cure. Instead, they are formulated to cross-polymerize into a very thin film of plastic-like material when exposed to sufficient amounts of UV light energy. Upon cross-polymerization, this film coheres firmly to the substrate. UV ink is very durable and also provides a means to produce a variety of special visual effects that generally may not be obtained using other types of inks. For these reasons, UV inks are often desirable. On the other hand, cured UV ink also adheres tightly to any other nearby surface during its curing phase, including impression cylinders and the many layers of ink previously applied to impression cylinders during normal operation.
In the past, conventional ink build-up has been removed from impression press cylinders via chemical, mechanical, and/or manual means. Commercially available cleaning systems that are compatible with conventional inks depend on the use of volatile solvents.
Due to UV ink's durability and cohesive nature, the removal of the cured ink from cylinder surfaces has proven to be more tedious than the removal of conventional or hybrid inks. The use of solvents alone has varying effectiveness for removing cured UV ink from impression cylinders, so press operators often apply mechanical or manual means. Specifically, manual methodology consists of scraping the ink from the cylinder with a hand-held blade often combined with the use of aggressive volatile solvents. This process is slow and fairly tedious to perform due to limited cylinder access and restrictive press configurations. It is also fairly expensive due to the associated press down-time, requires undesirable exposure to potentially harmful chemicals and hazardous pinch points, and cleaning results often vary according to operator skill, effort, and time restraints.
Due to the design and construction of a printing press, manual cleaning is uncomfortable to perform and also presents dangerous pinch-point hazards to the maintenance personal involved with cleaning these cylinders. During cleaning, the operator must work with his hands inside a powered-up press. The phrase “pinch-point” is a bit of an understatement here since a person caught in such a “pinch-point” can potentially be pulled into the press and maimed or killed.
Like many industrial machines, printing presses are often operated around the clock. During normal operation, it is often desirable to clean the press cylinders after each job or at least on a daily basis. Because of the issues mentioned above, however, cylinder cleaning may occur only once a week or so and often a cleaning is commenced only when necessary to produce acceptable product.